Among currently employed processes for synthesizing acetic acid one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
An improvement in the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in commonly assigned U.S. Pat. Nos. 5,001,259, issued Mar. 19, 1991; 5,026,908, issued Jun. 25, 1991 and 5,144,068, issued Sept. 1, 1992 and European patent 161,874 B2, published Jul. 1, 1992. As disclosed therein acetic acid is produced from methanol in a reaction medium comprising methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically effective concentration. The invention therein resides primarily in the discovery that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 4 weight (wt) % or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt % or 15 wt % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium, at least a finite concentration of water, methyl acetate and methyl iodide, a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium 0 contains water in concentrations as low as about 0.1 wt %, so low that it can broadly be defined simply as "a finite concentration" of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. resistance to catalyst precipitation, especially during the product recovery steps of the process wherein distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Pat. Nos. 5,001,259; 5,026,908 and 5,144,068 are herein incorporated by reference.
It has been found that a low water carbonylation process for the production of acetic acid reduces such by-products as carbon dioxide and propionic acid. However, the amount of other impurities, present generally in trace amounts, is also increased, and the quality of acetic acid sometimes suffers when attempts are made to increase the production rate by improving catalysts, or modifying reaction conditions. These trace impurities affect quality of acetic acid, especially when they are recirculated through the reaction process. Among the impurities which decrease the permanganate time of the acetic acid are carbonyl compounds, unsaturated carbonyl compounds, and organic iodides. As used herein, the phrase "carbonyl" is intended to mean compounds which contain aldehyde or ketone functional groups which compounds may or may not possess unsaturation.
The present invention is directed to removal of permanganate reducing compounds (PRC's) such as acetaldehyde which leads to formation of unsaturated aldehydes and other carbonyl impurities such as acetone, methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde and the like, and the aldol condensation products thereof. Other PRC's include alkyl iodides such as ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, and the like. Still other PRC's include propionic acid, a by-product of the acetic acid process.
PRC's typically have boiling points very close to those of iodide catalyst promoters (e.g., Mel) and it is difficult to sufficiently remove alkyl iodides. It is desirable to remove alkyl iodides from the reaction product since traces of these impurities (in the acetic acid product) tend to poison the catalyst used in the production of vinyl acetate, the product most commonly produced from acetic acid. The present invention is thus also directed to removal of alkyl iodides, in particular C.sub.2-12 alkyl iodides compounds. The carbonyl impurities may further react with iodide catalyst promoters to form multi-carbon alkyl iodides, e.g., ethyl iodide, butyl lo iodide, hexyl iodide and the like. Since many impurities originate with acetaldehyde, it is therefore a primary objective to remove or reduce the acetaldehyde and alkyl iodide content in the reaction system.
Conventional techniques to remove impurities include treatment of acetic acid with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like, which treatment may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the final product. It is known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine which reacts with the carbonyl compounds to form oximes followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the final product adds cost to the process and it has been found that distillation of the treated acetic acid product can result in additional impurities being formed.
While it is possible to obtain acetic acid of relatively high purity, the acetic acid product formed by the above described low water carbonylation process and purification treatment, frequently remains deficient with respect to the permanganate time. This is due to the presence therein of small proportions of residual impurities. Since a sufficient permanganate time is an important commercial test which the acid product must meet for many uses, the presence therein of such impurities that decrease permanganate time is objectionable. The removal of minute quantities of these impurities from the acetic acid by conventional treatment and distillation techniques is not economically or commercially feasible by distillation since the impurities have boiling points close to that of the acetic acid product.
It is important to determine where in the carbonylation process impurities can be removed. It is also important to determine by what economically viable process impurities can be removed without risk of further contamination to the final product or unnecessary added costs.
JP patent application 5-169205 discloses a method for manufacture of high purity acetic acid by adjusting the acetaldehyde concentration of the reaction solution below 1500 ppm. By maintaining the acetaldehyde concentration in the reaction solution below 1500 ppm, it is stated that it is possible to suppress the formation of impurities and manufacture high purity acetic acid by performing only basic distillation operations during purification of the crude acetic acid formed.
EP 487,284, B1, published Apr. 12, 1995, states that carbonyl impurities present in the acetic acid product generally concentrate in the overhead from the light ends column. Accordingly, the light ends column overhead is treated with an amine compound i.e., hydroxylamine which reacts with the carbonyl compounds to allow such carbonyls to be separated from the remaining overhead by distillation, resulting in an acetic acid product which has improved permanganate time.
EP 0 687 662 A2 describes a process for producing high purity acetic acid whereby an acetaldehyde concentration of 400 ppm or less is maintained in the reactor by removal thereof using a single or multi-stage distillation process. Streams suggested for processing to remove acetaldehyde include a light phase comprising primarily water, acetic acid and methyl acetate; a heavy phase comprising primarily methyl iodide, methyl acetate and acetic acid; an overhead stream comprising primarily methyl iodide and methyl acetate; or a recirculating stream comprising the light and heavy phase combined. Although four streams are suggested for processing, the reference teaches and exemplifies use of the heavy phase. No teaching or suggestion is given regarding which stream(s) possesses the greatest concentration of acetaldehyde.
Also disclosed in EP'662 is management of reaction conditions to control the formation of acetaldehyde in the reactor. By controlling the formation of acetaldehyde, it is stated that reduction of by-products such as crotonaldehyde, 2-ethylerotonaldehyde, and alkyl iodides are reduced. However, it is pointed out that management of reaction conditions "have a defect to increase a by-production speed of propionic acid." indicating that propionic acid is a problem with the disclosed process of '662.
Hence, EP'662 describes optimization of reaction conditions to avoid formation of acetaldehyde as well as removal of any acetaldehyde beyond a level of 400 ppm formed in the reactor.
While the above-described processes have been successful in removing carbonyl impurities from the carbonylation system and for the most part controlling acetaldehyde levels and permanganate time problems in the final acetic acid product, further improvements can still be made. There remains a need to determine where in the carbonylation process the permanganate reducing compounds, and in particular, acetaldehyde and alkyl iodides are most concentrated and therefore can be removed so as to insure consistent purity of product. At the same time, there remains a need to provide a process for removal of such carbonyl materials and iodide compounds without sacrificing the productivity of the carbonylation process or without incurring substantial additional operating costs.